Many construction machines are hydraulically driven. Examples of hydraulically driven construction machines include hydraulic shovels. Generally, in hydraulic shovels, a shovel is driven, an upper-part turning body is caused to turn, and a lower-part traveling body is caused to travel using a hydraulic actuator (hydraulic cylinders and hydraulic motors). In general, a hydraulic pressure supplied to a hydraulic actuator is often generated by a hydraulic pump whose drive source is an engine. In this case, the output of the hydraulic actuator is determined by the output of the engine.
A full capacity of the engine is not always required and, for example, 90% or 80% of the capacity is often sufficient for the operations of hydraulic shovels. Therefore, the operation mode of the hydraulic shovel is changed according to work loads so as to optimize engine output control for each of different work loads, thereby efficiently driving the engine to improve fuel consumption.
For example, it is made possible to set different operation modes such as “a high load mode” for performing a load operation corresponding to the maximum output of the engine, “a normal load mode” for performing a normal-load operation, and “a low load mode” for performing a light-load operation. Then, iso-horsepower control is performed so as to equalize the drive torque required by the hydraulic pump to drive the hydraulic actuator with the output torque of the engine in each operation mode, thereby efficiently utilizing the output of the engine to improve fuel consumption.
In recent years, there is a demand for reduction in the amount of fuel consumed by the engine. Simple reduction in the size of the engine would result in insufficient hydraulic output at the time of driving in the “high load mode.” Therefore, so-called hybrid hydraulic shovels have been developed that include an engine, a generator driven by the engine, a battery charged by the generator, and an electric motor driven with the electric power of the battery.
In common hybrid hydraulic shovels, the driving of the engine is controlled so that the number of revolutions of the engine is always constant. The torque of the engine is small during driving in the low load mode (low load state) for swinging an arm or a bucket. Therefore, in the low load mode, an extra torque is generated by increasing the torque of the engine while keeping the number of revolutions of the engine constant, and the generator is driven with the extra torque to generate electric power, thereby charging the battery.
On the other hand, in the high load mode (high load state) where the drive torque required by the hydraulic pump is greater than the rated output point of the engine, the torque of the engine is increased while the number of revolutions of the engine is kept constant, and the output of the electric motor is added (for assisting) to the output of the engine by driving the electric motor with the electric power from the battery.
Further, a construction machine is proposed that increases the torque of an engine to improve the rate of fuel consumption and drives a generator with a resulting extra torque to charge a battery at the time of a low load mode (at the time of a mode where the engine torque at the intersection point of the iso-horsepower line of the necessary horsepower and the governor characteristic line of the engine is smaller than the rated output of the engine). (For example, see Patent Document 1.)
According to such a circular engine fuel consumption rate characteristic as described above, the injection pressure is determined depending on the number of revolutions of the engine. Further, in the drive control of the engine, at the time of the low load mode, the fuel consumption rate, which is determined to be around the center of a circle where the number of revolutions at the time of the low load mode is best, is expected to improve slightly, but at the time of the high load mode, where the engine is driven in a region of high fuel consumption rates, no improvement is expected.
[Prior Art Document]
[Patent Document]
[Patent Document 1] Japanese Laid-Open Patent Application No. 2004-100621